In general, gas chromatograph mass spectrometers (which are hereinafter called the “GC-MS”) need to regularly or irregularly undergo adjustment and calibration processes using standard samples in order to maintain high levels of accuracy and sensitivity for measurements. A popularly used standard sample for the adjustment of a mass spectrometer in a GC-MS is PFTBA (perfluorotributylamine) which is comparatively inexpensive as well as easy to handle.
Patent Literature 1 discloses a configuration of a standard sample supply unit in a conventionally and commonly used. GC-MS, which is used for introducing a standard sample, such as PFTBA, into the mass spectrometer in place of a sample gas eluted from the column of the gas chromatograph. Specifically, the GC-MS described in the document includes a container for holding a solution of the standard sample, such as PFTBA, and a standard sample introduction pipe one end of which connected to the upper portion of the container via a solenoid valve. The other end of the pipe is either directly connected to the ion source, or connected via a T-joint to a sample supply passage which connects the exit port of the column and the ion source.
In a GC-MS, the ion source of the mass spectrometer is placed within an analysis chamber maintained in a high-vacuum state by evacuation with a vacuum pump. Therefore, the ion source and the sample supply passage connected to it are also internally maintained at a reduced pressure level. Accordingly, in the aforementioned standard sample supply unit, when the solenoid valve connected to the standard sample container is opened, a standard sample gas resulting from vaporization of the standard sample held in the container is drawn through the standard sample introduction pipe into the ion source. In the adjustment process of the mass spectrometer, while the standard sample gas is introduced into the ion source in this manner, either a scan measurement, or a selected ion monitoring (SIM) measurement in which a standard sample component is selected as the target, is performed to acquire data.
In a typical procedure for adjusting a mass spectrometer in a GC-MS, voltages applied to such elements as the lens electrode for converging ions are adjusted so as to maximize the area value (or height value) of the peak corresponding to the PFTBA on the obtained mass chromatogram, i.e. so as to maximize detection sensitivity. Voltages applied to the quadrupole mass filter are also adjusted so that the pattern of the peaks appearing on the mass spectrum corresponding to the PFTBA becomes as similar as possible to that of a standard mass spectrum previously prepared for PFTBA. Subsequently, a voltage applied to the ion detector, such as a secondary electron multiplier, is adjusted so that the actual signal value, such as the peak area value corresponding to the PFTBA on the mass chromatogram, will match with a reference value. By adjusting the applied voltage to the ion detector, i.e. the gain of the ion detector, in this manner so that the real signal value for a standard sample of a known concentration matches with the reference value, it is possible improve the reproducibility of the measurement.
However, the previously described type of conventional GC-MS has the following problem.
The introduction amount of PFTBA drawn from the standard sample container into the ion source (per unit time) during the adjustment of the mass spectrometer is considerably dependent on not only the gas pressure (degree of vacuum) within the ion source but also the surrounding temperature (i.e. ambient temperature), since the standard sample container is placed under ambient temperature and the volatilization volume of the PFTBA considerably varies with the ambient temperature. A variation in the amount of introduction of the PFTBA into the ion source causes a considerable fluctuation in the amount of ions generated from the PFTBA within the ion source, which consequently changes the actually obtained signal value even with the same voltage applied to the ion source. As a result, as in the case of the summer and winter seasons, when there are large seasonal differences in the ambient temperature, the gain of the detector becomes also different and causes a variation in the sensitivity despite the effort of adjusting the applied voltage to make the actual signal value match with the same reference value. Therefore, for example, it will be difficult to accurately compare a measured result obtained in the summer season with one obtained in the winter season.